In optical transmission systems and especially in systems having long unrepeatered fiber spans, it is important to launch as high an optical power into the transmission fiber as possible. Unfortunately, the amount of launch power usable at a particular wavelength is limited by nonlinear phenomena associated with the transmission medium. Such nonlinear phenomena include stimulated Brillouin scattering ("SBS"), stimulated Raman scattering, self-phase modulation, four-photon mixing and cross-phase modulation. These phenomena can degrade the optical signals and increase bit error rates for the data transported by the transmission system.
SBS is a nonlinear process requiring close attention in transmission systems employing silica fibers, narrow linewidth optical sources (typically, less than one megahertz), and operating between 1.0 .mu.m and 1.6 .mu.m because the Brillouin linewidth in the corresponding wavelength range is between 15 MHz and 40 MHz. SBS can be significant for power levels as low as 1 mW in single-mode optical fibers. The efficiency for SBS decreases as the linewidth of the optical source is increased. Consequently, artificial broadening of the spectrum of the optical source through optical modulation, for example, serves as a means of increasing the SBS threshold. The SBS power threshold is arbitrarily defined as the input optical pump signal power level at which the power of the input optical pump signal becomes equal to the power of the backward Stokes signal.
Both direct frequency modulation ("FM") and external phase modulation ("PM") have been demonstrated as effective means of SBS suppression in lightwave systems. Both modulation techniques introduce a predetermined amount of spectral broadening to the transmitted signals.
The direct FM approach uses a dither signal on the laser bias to provide large frequency excursions, usually on the order of 10 GHz. Through the use of this technique, the SBS threshold has been increased as much as 15 dB. However, direct FM of an injection laser also results in substantial amplitude modulation ("AM"), called residual AM, that degrades system performance.
Phase modulation suppresses SBS and avoids the production of residual amplitude modulation. Using an external optical phase modulator driven with a single-frequency sinusoidal signal, it is possible to attain a 5 dB increase in the SBS threshold. It has proven impractical to increase the SBS threshold further by using PM with a high modulation index because the required modulator RF drive power very quickly increases to an unacceptably high level.